This invention relates to guides for r.f. magnetic flux.
Flux guiding or flux concentration is conventionally performed using a ferrite core. Ferrites are not frequency selective, and that can be a benefit in some cases, for example a radio aerial, but in other cases, such as the IF stage of a radio, it is a disadvantage.
Microstructured materials have been proposed (IEEE Transactions on Microwave Theory and Techniques, 1999, 47, 2075–2084, Magnetism from Conductors and Enhanced Non-Linear Phenomena by J B Pendry, A J Holden, D J Robbins and W J Stewart, and International Patent Application WO 00/41270), in which the magnetic permeability depends on the frequency of oscillation of the magnetic field i.e. depends on the r.f. frequency.
Such materials comprise a structure with magnetic properties comprising an array of capacitive elements, the array exhibiting a predetermined magnetic permeability in response to incident electromagnetic radiation lying within a predetermined frequency band, wherein each element includes a low resistance conducting path and is such that a magnetic component of the electromagnetic radiation lying within the predetermined frequency band induces an electrical current to flow around said path and through said associated element, and wherein the size of the elements and their spacing apart are selected such as to provide a predetermined permeability in response to said received electromagnetic radiation. At least one dimension of each capacitive element is preferably less than the wavelength of the electromagnetic radiation. The capacitive elements may be in the form of conductive sheets wound as a spiral, or a plurality of stacked planar sections each of which is electrically isolated from each other and is in the form of a spiral. The further preferred features of the materials are disclosed and claimed in International Patent Application WO 00/41270, the contents of which are incorporated herein by reference.
Typically such materials consist of an array of capacitive elements at least one of whose dimensions is much smaller than the wavelength at which the desired permeability is exhibited, in which resonant interaction between the inductance of the structure and the capacitive elements causes electromagnetic energy to be shared between the magnetic fields and the electrostatic fields within the capacitive structure.